It is known to introduce cellulose into an aqueous NMMO solution and to precipitate cellulosic products from that solution by a spinning process or the like. The cellulose is introduced into the aqueous NMMO solution and the solution thus forms a homogeneous cellulose solution. The films or shaped-cellulose objects are formed by precipitation of the cellulose from the solution in water. The cellulosic products are those which have generally been made heretofore by the viscose filament process.
Spinnable solutions of cellulose in aqueous NMMO have a substantial advantage over viscose in that the NMMO for the spinning bath can be recovered and thus does not constitute an environmental contaminant; furthermore, the process is characterized by lack of sulfur-containing emissions.
To regenerate the NMMO contained in a depleted spinning bath or to reuse the NMMO for the production spinnable cellulose solutions, the spinning bath solution must be cleaned and concentrated.
Complete cleaning by prior art processes must involve the following steps:
(A) Decoloration
Upon the evaporation of water for concentrating the NMMO in dilute aqueous NMMO solutions, because of the reaction of NMMO with cellulose decomposition products, strongly yellow to brown coloration occurs.
This coloration appears to be the result of the formation of pigmentatious compounds from polyvalent phenols from decomposition products to cellulose and from NMMO stabilizers which are customarily added to the solution. Due to increasing coloration of the NMMO solution, it is not possible to bleach the solution to such an extent as to obtain shaped cellulosic bodies with the desired degree of whiteness.
(B) Removal of transition metals
Transition metals, usually iron, are generally entrained into the process circulation by corrosion in the circulating piping, etc. and from the cellulose itself. The content of the transition metals must not exceed a certain critical maximum since the presence of the transition metals tends to lower the initial temperature for spinning mass deflagration.
When the stabilizer is gallic acid propylester, anionic metal complexes are formed which can be removed by anion exchangers.
If the stabilizer is, for example, rutin, an iron complex can form which cannot be removed by ion exchangers. As a consequence, the iron concentration tends to rise in the process circulation what increases the safety hazards. As a consequence, it is important to provide for the removal of iron and other transition metal ions from the process.
(C) Removal of nitrosoamines
In fresh NMMO, as a result of its fabrication, nitrosoamines may be present. The nitrosoamines can have a variety of toxic effects since it is highly detrimental to the liver, possesses gene toxicity in vitro as well as in germ cells, produces cancer in somatic cells, etc. Because of the general tumor-inducing effect of nitrosamines, complete removal thereof is essential for worker safety and health.
(D) Removal of cloudiness or turbidity
In addition to the discoloration of the spinning bath, precipitation can occur therein in the form of extremely finely-divided cellulosic materials and alkali and alkaline earth salts. These turbidities, which increase in the solution during repeated use in recycling, cannot be removed by filtration without filter aids. They affect detrimentally the product quality, give rise to problems when performing in-line color measurements and must, therefore, be removed.
In the conventional cleaning processes, moreover, it is important to avoid losses in NMMO.
Conventional cleaning processes involve a number of methods which have some basic drawbacks:
(a) Cleaning utilizing anion exchangers
This method is limited to decoloration by removal of ion complexes. Iron or other transition metals which are in ionic form, can be removed with an effectiveness which depends upon the stabilizer system. Significant removal of the fine cellulosic precipitate, i.e. the turbidity is not possible. Relatively large amounts of regenerating chemicals are required.
(b) Recrystallization from acetone
This method is highly time-consuming and energy expensive. Furthermore, the NMMO recovery rate is only a maximum of 85%.